Lead shot for waterfowl hunting has been banned in the US for a few decades. It's still used for upland bird hunting. I think it's still frequently used illegally for waterfowl.

Not an expert, but have a decent layman's understanding. Could totally be wrong about the next part:

It's my understanding that lead contamination of wild animals through hunting primarily occurs due to various sizes of bird shot. The greater surface area allows a much higher level of contamination. It also forms lead dust in the shell from friction and when fired. It's also easier for animals to eat it. Rifle rounds and slugs are fairly inert as the larger size prevents most consumption and less absorption when it is consumed.